Power-unit support structure of motorcycle

ABSTRACT

A power-unit support structure of a motorcycle having a power unit and a frame structure including a tubular member having a hollow rear end portion, comprising a rigid bracket member secured to the tubular member and positioned in the vicinity of the rear end portion of the tubular member, a first pivot assembly positioned between the power unit and the tubular member and pivotally movable about an axis fixed with respect to the frame structure, a second pivot assembly mounted on the power unit and pivotally movable about an axis fixed with respect to the frame structure and parallel with the axis of pivotal motion of the first pivot assembly, rigid connecting members providing connection between the first and second pivot assemblies, a damper assembly rigidly connected to the first pivot assembly and elastically engaging the rear end portion of the tubular member. Such a the damper assembly includes an arm member rigidly connected at one end to the first pivot assembly and terminating in conjunction with the rear end portion of the tubular member and an elastic damper element engaging the rear end portion of the tubular member, the arm member being coupled at the other end to the elastic damper element.

FIELD OF THE INVENTION

The present invention relates to a motorcycle and, more particularly, toa power-unit support structure for use in a motorcycle such as apower-driven bicycle, a scooter or a power-driven tricycle.

BACKGROUND OF THE INVENTION

An existing motorcycle has a power unit mounted on the frame structureof the vehicle by means of a swing-arm support structure. With such asupport structure, the power unit which consists of an engine and apower transmission mechanism is suspended from the frame structure ofthe vehicle in such a manner as to form part of a swing-arm structurewhich is swingable with respect to the frame structure. The swing-armpower-unit support structure includes a suspension assembly whichintervenes between the power unit and the rearmost end of a downtubewhich forms part of the frame structure of the vehicle. The suspensionassembly includes a rigid link member pivotally coupled at one end tothe power unit and at the other to the downtube with an elastic damperelement interposed between the rear end of the downtube and the linkmember. The link member has upper and lower projections at its endconnected to the downtube.

The power unit supported by means of such a suspension assembly issubjected not only to impacts transferred from the road wheels throughthe frame structure but also to vibrations created in the power unit perse. The impacts transferred through the frame structure cause the powerunit to oscillate vertically with respect to the frame structure and thevibrations originating in the power unit tend to be transmitted throughthe link member to the downtube. The oscillatory motions of the frontend portions of the power unit cause the link member to rock verticallywith respect to the downtube. Under such conditions, the elastic damperelement provided between the downtube and the link member attenuatessuch oscilatory motions of the power unit and absorbs the vibrations tobe transmitted through the link member to the downtube. As a result ofthe rocking motions of the link member, the elastic damper element issubjected to compressive forces alternately applied from the upper andlower projections of the link member. In this instance, the distancebetween the axis about which the link member is caused to rock and thearea in which the compressive forces are concentrated in the elasticdamper elements is limited by the diameter of the downtube as will bediscussed in more detail. To enable the elastic damper element toattenuate the oscillating motions of the link member satisfactorily, theelastic damper element is required to provide a relatively large springconstant. Such a damper element is however not adapted to dampen thevibrations originating in the power unit per se and, thus, thevibrations created in the power unit are allowed to transmit through theelastic damper element to the frame structure of the motorcycle in asignificant proportion.

It is, accordingly, a prime object of the present invention to providean improved motorcycle power-unit support structure which is capable ofnot only attenuating the oscillatory motions of the power unit butabsorbing the vibrations which originate in the power unit per se.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a power-unitsupport structure of a motorcycle having a power unit and a framestructure including a support member having a rear end portionterminating in conjunction with the power unit, comprising a rigidbracket member secured to the support member and positioned rearwardlyof the support member; a pivot assembly positioned between the powerunit and the rear end portion of the support member and pivotallymovable about an axis substantially fixed with respect to the framestructure; rigid connecting means providing connection between the pivotassembly and the power unit; damper means rigidly connected to the pivotassembly and elastically engaging the rear end portion of the supportmember, the damper means including a rigid arm member rigidly connectedat one end thereof to the pivot assembly and terminating in conjunctionwith the rear end portion of the support member and an elastic damperelement engaging the rear end portion of the support member, the armmember being coupled at the other end thereof to the elastic damperelement. The aforesaid pivot assembly constituting a first pivotassembly, the support structure may further comprise a second pivotassembly mounted on the power unit and pivotally movable about an axissubstantially fixed with respect to the frame structure andsubstantially parallel with the axis of pivotal motion of the firstpivot assembly, the rigid connecting means providing connection betweenthe first and second pivot assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawbacks of a prior-art motorcycle power-unit support structure andthe features and advantages of a power-unit support structure accordingto the present invention will be more clearly appreciated from thefollowing description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a side elevation view of a representative example of a knownpower-unit support structure for a motorcycle;

FIG. 2 is a side elevation view of a motorcycle having incorporatedtherein a power-unit support structure embodying the present invention;

FIG. 3 is a horizontal sectional view showing, to an enlarged scale, afirst preferred embodiment of a power-unit support structure accordingto the present invention;

FIG. 4 is a side elevation view of the power-unit support structureillustrated in FIG. 3;

FIGS. 5A to 5B are views showing the configuration of an elastic damperelement included in the power-unit support structure illustrated inFIGS. 2 and 3, wherein FIG. 5A is a top plan view of the damper element,FIG. 5B is a side elevation view of the damper element as viewed from avertical plane indicated by line B--B in FIG. 5A, FIG. 5C is a frontelevation view of the damper element as viewed from a vertical planeindicated by line C--C in FIG. 5A, and FIG. 5C is a horizontal sectionalview taken on a plane indicated by line D--D in FIG. 5C;

FIG. 6 is a horizontal top plan view showing a second preferredembodiment of a power-unit support structure according to the presentinvention; and

FIG. 7 is a cross sectional view taken on vertical planes indicated bylines VII--VII in FIG. 6.

FURTHER DESCRIPTION OF THE PRIOR ART

Description will be further made regarding a known swing-arm power-unitsupport structure of a motorcycle. In FIG. 1 of the drawings is shown anexample of such a power-unit support structure, wherein the power unitconsisting of an internal combustion engine and a power transmissionmechanism is generally indicated at 10. As shown, the power unit 10 issuspended from the frame structure 12 of the motorcycle by means of aspring damper cylinder 14 and a front suspension assembly 16. The springdamper cylinder 14 is connected at one end to a lengthwise middleportion or a rear end portion of the power unit 10 and at the other to ahorizontally extending rear end portion of the frame structure 12. Thefront suspension assembly 16 intervenes between the foremost end of thepower unit 10 and the rearmost end of a hollow downtube 18 which formspart of the frame structure 12. The downtube 18 has securely attachedthereto a rigid front bracket member 20 and, likewise, the power unit 10has a rigid rear bracket member 22 located at the foremost end of thepower unit in conjunction with the bracket member 20 on the downtube 18.A rigid link member 24 is pivotally coupled at one end to the frontbracket member 20 by a pivot pin 26 and at the other to the rear bracketmember 22 by a pivot pin 28. The link member 24 has upper and lowerprojections 24a and 24b at its end connected to the front bracket member20. The upper and lower projections 24a and 24b protrude indiametrically opposite directions away from the pivot pin 26 on thefront bracket member 20. An elastic damper element 30 interposed betweenthe rear end of the downtube 18 and these projections 24a and 24b of thelink member 24. The elastic damper element 30 is constructed typicallyof rubber.

During running of the motorcycle with the power unit 10 in operation,the power unit 10 is subjected to impacts transferred from the roadwheels through the frame structure 12 and vibrations created in thepower unit per se. The impacts transferred through the frame structure12 to the power unit 10 cause a front end portion of the power unit 10to oscillate vertically with respect to the frame structure 12, whilethe vibrations originating in the power unit 10 are transmitted throughthe link member 24 to the front bracket member 20. The oscillatorymotions of the front end portions of the power unit 10 in turn cause thelink member 24 to rock vertically about the center axis of the pivot pin26 on the front bracket member 20. The elastic damper element 30provided between the downtube 18 and the link member 24 of the frontsuspension assembly 16 as described above serves to attenuate suchoscillatory motions of the front end portion of the power unit 10 and toabsorb the vibrations to be transmitted through the link member 24 tothe downtube 18.

The rocking motions of the link member 24 about the center axis of thepivot pin 26 result in rocking motions of the upper and lowerprojections 24a and 24b of the link member 24. The elastic damperelement 30 is thus subjected to compressive forces alternately from theupper and lower projections 24a and 24b of the link member 24 as thelink member 24 is caused to rock about the center axis of the pivot pin26 on the front bracket member 20. These compressive forces produced inthe elastic damper element 30 are concentrated in an annular areaaligned with the circular rear end edge of the downtube 18 against whichthe damper element 30 is pressed. The distance d between such an areaand the center axis of the pivot pin 26 about which the link member 24is caused to rock as above discussed is limited by the diameter of thedowntube 18. The upper and lower projections 24a and 24b of the linkmember 24 are for this reason permitted to rock with respect to theframe structure 12 through angles limited by the diameter of thedowntube 18. To provide desired oscillation damping characteristics withuse of such a suspension assembly 16, it is thus required to use arelatively hard elastic material for the elastic damper element 30 toprovide a relatively large spring constant (load rate). The elasticdamper element 30 having such a large spring constant is howeverinappropriate for the attenuation of the vibrations originating in thepower unit 10 per se. It is for this reason practically inevitable inthe suspension assembly 16 of the described construction that thevibrations created in the power unit 10 be allowed to transmit throughthe elastic damper element 30 to the frame structure 12 of themotorcycle. The present invention contemplates elimination of such adrawback of a prior-art support structure for the power unit of amotorcycle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be hereinafter made regarding the construction andarrangement of preferred embodiments of a power-unit support structureaccording to the present invention. For purposes of description, it willbe assumed that a power-unit support structure according to the presentinvention is embodied in a scooter illustrated in FIG. 2.

As shown in FIG. 2, the scooter incorporating the power-unit supportstructure embodying the present invention has a steering front roadwheel 32 and a driving rear road wheel 34. The front and rear roadwheels 32 and 34 are connected together by means of a frame structure 36coupled to a front fork structure 37 carrying the front road wheel 32.The frame structure 36 includes a hollow tubular member 38 having alower rear end portion horizontally extending away from the front roadwheel 32 toward the rear road wheel 34. The hollow tubular member 18 isherein called downtube and is assumed to have a circular cross sectionand has its rear end located in front of a power unit which is generallyindicated at 40. The power unit 40 consists of an internal combustionengine and a power transmission mechanism which operatively connects theengine to the driving rear road wheel 34 as is well known in the art. Afirst preferred embodiment of a power-unit support structure accordingto the present invention, as indicated generally at 42, is adapted tohave such a power unit 40 suspended from the frame structure 36 and maybe provided in combination with a spring damper cylinder which is partlyseen at 44 in FIG. 2. As discussed in connection with the prior-artpower-unit support structure illustrated in FIG. 1, the spring dampercylinder 44 is connected at one end to a lengthwise middle portion or arear end portion of the power unit 40 and at the other to a horizontallyextending rear end portion of the frame structure 36. The constructionand arrangement of such a spring damper cylinder 44 is well known in theart and is rather immaterial to the understanding of the subject matterof the present invention and, for these reasons, will not be hereindescribed in more detail.

The power-unit support structure 42 embodying the present invention thusintervenes between the foremost end of the power unit 40 and therearmost end of the downtube 46 and comprises a pair of rigid bracketmembers 46 and 46' which are welded or otherwise securely attached to arear end portion of the downtube 38. The bracket members 46 and 46' arespaced apart in parallel from each other laterally of the framestructure 36 (FIG. 2) of the vehicle in front of the power unit 40.These bracket members 46 and 46' may be constructed separately of eachother or, otherwise, may form part of a unitary rigid structure securedto the downtube 38. If desired, furthermore, the bracket members 46 and46' may be combined with rigid reinforcement members 48 and 48' whichare also welded or otherwise securely attached to the rear end portionof the downtube 38. These reinforcement members 48 and 48' may also beconstructed separately of each other and respectively connected to thebracket members 46 and 46' or otherwise may form part of a unitary rigidstructure secured to the downtube 38.

The bracket members 46 and 46' are formed with circular openings 50 and50', respectively, which are aligned with each other laterally of theframe structure 36 of the vehicle. The openings 50 and 50' thus formedin the bracket members 46 and 46' are provided to have a first pivotaldamper assembly 52 connected to the downtube 38 through the bracketmembers 46 and 46' and pivotally movable with respect to the framestructure 36 of the vehicle about an axis in a lateral direction of theframe structure 36. In the embodiment of the present invention as shownin FIG. 3, the first pivotal damper assembly 52 comprises a rigidelongated member constituted by a bolt 54 which has a head portion and athreaded end portion respectively at the opposite ends thereof and astem portion which extends between these head and threaded end portions.The bolt 54 is passed through the openings 50 and 50' with its stemportion extending between the bracket members 46 and 46' and is fastenedto the bracket members with its head portion forced against the outerface of one bracket member 46 and with a nut 55 tightened to thethreaded end portion of the bolt 52 and forced against the outer face ofthe other bracket member 46'. The bolt 54 thus arranged extendshorizontally in a lateral direction of the frame structure 36 of thevehicle and has its stem portion located immediately at the rear of thedowntube 38 as shown. The stem portion of the bolt 54 is coaxiallyreceived in a rigid inner tubular member constituted by an inner sleevemember 56. The inner sleeve member 56 extends between the bracketmembers 46 and 46' with its opposite end faces held in contact with theinner faces of the bracket members 46 and 46', respectively. The innersleeve member 56 in turn is coaxially received in a pair of cylindricalelastic damper elements constituted by rubber bushings 58 and 58' whichare located on opposite end portions, respectively, of the inner sleevemember 56 and which are slightly spaced apart from the inner faces ofthe bracket members 46 and 46', respectively. The rubber bushings 58 and58' are closely fitted in rigid hollow cylindrical members constitutedby metal collars 60 and 60', respectively. The metal collars 60 and 60',in turn, are coaxially received jointly in a rigid outer tubular memberconstituted by an inner sleeve member 62. The outer sleeve member 62extends between the bracket members 46 and 46' with its opposite endfaces slightly spaced apart from the inner faces of the bracket members46 and 46', respectively, as shown. The rear end portion of the downtube38 is rearwardly open immediately in front of a lengthwise middleportion of the outer sleeve member 62 of the first pivot assembly 52thus constructed and arranged.

The power-unit support structure 42 embodying the present inventionfurther comprises a second pivot assembly 64 connected to the the powerunit 40 and pivotally movable with respect to the frame structure 36 ofthe vehicle about an axis also in a lateral direction of the framestructure 36. The second pivot assembly 64 is coupled to the power unit40 through a pair of bracket members 66 and 66' securely attached to thebody structure of the power unit 40 and formed with axial bores alignedwith each other in a direction parallel with the first pivot assembly 52and each having a circular cross section. In the embodiment of thepresent invention as shown in FIG. 3, the second pivot assembly 64comprises a pair of outer rigid hollow cylindrical members constitutedby metal collars 68 and 68' coaxially received in the axial bores in thebracket members 66 and 66', respectively. These outer metal collars 68and 68' in turn have coaxially received therein cylindrical elasticdamper elements constituted by rubber bushings 70 and 70'. These rubberbushings 70 and 70' in turn have coaxially received therein rigid innercylindrical members constituted by inner metal collars 72 and 72',respectively. The second pivot assembly 64 further comprises a rigidelongated member constituted by a bolt 74 which has a head portion and athreaded end portion respectively at the opposite ends thereof and astem portion extending between these head and threaded end portions. Theinner metal collars 72 and 72' are located around opposite end portions,respectively, of the stem portion of this bolt 74. The bolt 74 is passedthrough the axial bores in the inner collars 72 and 72' with its headportion forced against the outer end face of one inner metal collar 72and with a nut 76 tightened to the threaded end portion of the bolt 74and forced against the outer end face of the other inner metal collar72'. The bolt 54 thus arranged extends horizontally in a lateraldirection of the frame structure 36 of the vehicle and has its stemportion in parallel with the stem portion of the bolt 54 of the firstpivot assembly 52. A rigid tubular member constituted by a sleeve member78 is loosely received coaxially on the stem portion of the bolt 74 andextends between the inner metal collars 72 and 72' with its opposite endfaces held in contact with the inner end faces of the collars 72 and72', respectively, as shown.

The power-unit support structure 42 embodying the present inventionfurther comprises a pair of rigid link members 80 and 80' extendingbetween the first and second pivot assemblies 52 and 64 and spaced apartin parallel from each other laterally of the frame structure 36 of thevehicle. Each of the rigid link members 80 and 80' is welded orotherwise securely coupled at one end to the outer sleeve member 62 ofthe first pivot assembly 52 and at the other to the sleeve member 78 ofthe second pivot assembly 64. These rigid link members 80 and 80' may bearranged so that the exhaust pipe which leads from the power unit 40 asindicated at 82 in FIG. 2 vertically extends between the link members 80and 80'.

As will be also seen from FIG. 4 of the drawings, the power-unit supportstructure 42 embodying the present invention further comprises a damperassembly 83 providing engagement between the first pivot assembly 52 andthe downtube 38. Such a damper assembly 83 is positioned within a rearend portion of the downtube 38 and comprises a rigid arm member 84 whichis welded or otherwise securely connected at one end to the outer sleevemember 62 of the first pivot assembly 52. The arm member 84 extendsforwardly and upwardly from the first pivot assembly 52 and thenforwardly and horizontally into the rear end portion of the downtube 38.The arm member 84 has its leading end thus located within the read endportion of the downtube 38 and has an elastic damper element 86 fittedto its leading end portion. As will be better seen from FIGS. 5A, 5B and5C of the drawings, the elastic damper element 86 comprises two, upperand lower lug portions 88 and 88' slightly spaced apart vertically fromeach other with a gap 90 formed therebetween and an intermediate portion92 which conjoins the lug portions 88 and 88' together. The intermediateportion 92 has a cross section having a circular portion and arectangular portion merging out of the circuit portion as will be seenfrom FIG. 5D. As indicated by broken lines in FIG. 3, the arm member 84is formed with a recess or cutout 94 shaped conformingly to such a crosssection of the intermediate portion 92 and the elastic damper element 86is fitted to the leading end portion of the arm member 84 with theintermediate portion 92 of the former closely received in this cutout 94in the latter.

Each of the upper and lower lug portions 88 and 88' of the elasticdamper element 86 is formed with a plurality of rib portions spacedapart in parallel from each other laterally of the frame structure 36 ofthe vehicle and each extending in a fore-and-aft direction of the framestructure 36. In FIGS. 5B and 5C, these rib portions are indicated at 96for the rib portions of the upper lug portion 88 and at 96' for the ribportions of the lower lug portion 88'. The rib portions 96 of the upperlug portion 88 protrude upwardly away from the gap 90 and, likewise, therib portions 96' of the lower lug portion 88' protrude downwardly awayfrom the gap 90. These rib portions 96 and 96' of the elastic damperelement 86 have their tops located on a cylindrical plane 98 having acenter axis coincident with the center axis of the rear end portion ofthe downtube 38. Such a cylindrical plane 98 has a predetermineddiameter which may be slightly larger than the inside diameter of therear end portion of the downtube 38. The rib portions 96 and 96' of theelastic damper element 86 are thus elastically pressed at their tops onthe inner peripheral surface of the rear end portion of the downtube 38.

The construction of the power-unit support structure 42 as abovedescribed is advantageous in that the distance (represented by D in FIG.3) between the center axis, viz, the axis of pivotal motion of the firstpivot assembly 52 and the center point of the pressure to act on theelastic damper element 86 from the arm member 84 can be selectedarbitrarily by selecting the length of the arm member 84.

While each of the lug portions 88 and 88' is shown formed with three ribportions, each lug portion of the elastic damper element 86 may bemodified in configuration to have only two or more than three ribportions which may be common in number to both of the two lug portionsor which may differ in number from one lug portion to the other. Ifdesired, only the upper lug portion 88 of the 86 may be formed with theribs 96 with the the lower lug portion 88' shaped to have its lower facearcuately curved in cross section on the above mentioned cylindricalplane 98. Such a configuration of the lower lug portion 88' is preferredto add to the spring constant of the lower lug portion 88' which is tobe subjected to greater downward loads than the upwardly directed loadsto be applied to the upper lug portion 88.

FIGS. 6 and 7 of the drawings show a second preferred embodiment of apower-unit support structure according to the present invention. Theembodiment herein shown is a modification of the embodiment hereinbeforedescribed and is assumed to comprise all the component members andelements of the first embodiment except for the single arm member 84 andthe elastic damper element 86 attached thereto. Thus, the embodimentshown in FIGS. 6 and 7 comprises first and second pivot assemblies 52and 64 and link members 80 and 80' which are entirely similar to theirrespective counterparts in the embodiment hereinbefore described.

In the embodiment shown in FIGS. 6 and 7, there are provided in additionto such pivot assemblies 52 and 64 and link members 80 and 80' a pair ofrigid arm members 100 and 100' each coupled at one end to the outersleeve member 62 of the first pivot assembly 52. These arm members 100and 100' are spaced apart in parallel laterally of the frame structure36 (FIG. 2) of the vehicle and extend forwardly from the first pivotassembly 52 through openings 102 and 102', respectively, provided in avertical wall member interconnecting the bracket members 46 and 46'secured to the downtube 38. The arm members 100 and 100' have leadingend portions extending in parallel with and on both sides of a rear endportion of the downtube 38.

The power-unit support structure shown in FIGS. 6 and 7 furthercomprises a pair of damper assemblies 104 and 104' providing elasticconnections from the arm members 100 and 100' to the downtube 38. Thesedamper assemblies 104 and 104' comprise rigid cantilever shafts 106 and106', respectively, projecting in opposite directions at right anglesfrom the rear end portion of the downtube 38 in a lateral direction ofthe frame structure 36. The two cantilever shafts 106 and 106' havecoaxially received thereon cylindrical elastic damper elements 108 and108', respectively. Each of these elastic damper elements 108 and 108'is coaxially received in a generally semicylindrical sleeve member 110which has an axial slot extending throughout the length of the sleevemember 110. The axial slot thus formed in each sleeve member 110 is openforwardly, viz., in a direction opposite to the first pivot assembly 52as will be better seen from FIG. 7 so that the sleeve member 110, whichis preferably constructed of spring steel, has its axial edge portionsvertically deformable toward and away from each other. Each of the abovedescribed arm members 100 and 100' forwardly terminate close to each ofthe cantilever shafts 106 and 106' and is securely connected at itsleading end to the sleeve member 110 thus forming part of each of thedamper assemblies 104 and 104'.

The power-unit support structure thus constructed is also advantageousin that the distance (represented by D') between the axis of pivotalmotion of the first pivot assembly 52 and the center point of thepressure to act on the elastic damper element 110 from each of the armmembers 100 and 100' can be selected arbitrarily by selecting the lengthof each arm member. The embodiment of FIGS. 6 and 7 is furtheradvantageous in that the two damper assemblies 104 and 104' positionedoutside the downtube 38 provide easy access thereto during assemblageand servicing of the power-unit support structure.

As will be understood from the foregoing description, each of theembodiments of a power-unit support structure according to the presentinvention as an advantage that the distance between the axis of pivotalmotion of the first pivot assembly and the center point of the pressureto act on the elastic damper element or each of the elastic damperelements from the arm member forming part of the damper assembly can beselected to have a sufficiently large value. To provide desiredoscillation damping characteristics with use of such a power-unitsupport structure, it is thus allowed to use a relatively soft elasticmaterial for each of the elastic damper elements included in the supportstructure. A power-unit support structure according to the presentinvention is for this reason capable of not only attenuating theoscillatory motions of the power unit but absorbing the vibrations whichoriginate in the power unit per se.

What is claimed is:
 1. A power unit support structure of a motorcyclehaving a power unit and a frame structure including a support memberhaving a tubular end portion terminating in conjunction with said powerunit, which comprises:an elongated swing member positioned between saidpower unit and said tubular end portion and having a first end thereofpivotally connected with said power unit; a pivot support assembly forpivotally supporting said elongated swing member on said support member;and an elastic damper element accommodated within said tubular endportion and coupled with a second end of said swing member.
 2. A powerunit support structure of a motorcycle as set forth in claim 1, in whichsaid pivot assembly includes a bracket member secured to said supportmember, a pin member supported on said bracket member and extending in alateral direction of said motorcycle, at least one elastic membercoupled with the periphery of said pin member, and a sleeve membercoupled on said elastic member and connected with said elongated swingmember.
 3. A power unit support structure as set forth in claim 1, whichfurther comprises an additional elastic damper element provided at thepivotal connection between said first end of said swing member and saidpower unit.
 4. A power-unit support structure as set forth in claim 1,in which said elastic damper element comprises two lug portions slightlyspaced apart from each other with a gap formed therebetween and anintermediate portion which conjoins the lug portion together, each ofthe lug portions being held in contact with the inner peripheral surfaceof the tubular end portion, said swing member being held in retainingengagement at said second end thereof with said intermediate portion. 5.A power-unit support structure as set forth in claim 4, in which atleast one of said lug portions is formed with a plurality of ribportions which are spaced apart from each other and which areelastically held in contact at their respective tops with the innerperipheral surface of the tubular end portion of said support member. 6.A power-unit support structure as set forth in claim 5, in which saidtubular end portion is cylindrical and said rib portions have their topslocated on a cylindrical surface having a center axis substantiallycoincident with the center axis of the tubular end portion of thesupport member.
 7. A power-unit support structure as set forth in claim6, in which said cylindrical surface has a predetermined diametersubstantially larger than the inside diameter of the rear end portion ofsaid support member.
 8. A power-unit support structure as set forth inclaim 5, in which said lug portions are spaced apart vertically fromeach other and in which said rib portions are spaced apart from eachother substantially horizontally.
 9. A power-unit support structure asset forth in claim 5, in which one of said lug portions is shaped tohave a surface portion arcuately curved in cross section on saidcylindrical plane, said surface portion being elastically held incontact with the inner peripheral surface of the tubular end portion ofsaid support member.
 10. A power-unit support structure as set forth inclaim 9, in which said lug portions are spaced apart vertically fromeach other and consist of upper and lower lug portions, the upper lugportion being formed with said rib portions and the lower lug portionbeing shaped to have a surface portion arcuately curved in cross sectionsaid cylindrical plane, said surface portion being elastically held incontact with the inner peripheral surface of the tubular end portion ofsaid support member.
 11. A power-unit support structure as set forth inclaim 1, in which said pivot support assembly comprises a rigidelongated member secured to a bracket member and having a stem portionwhich extends between the opposite ends of the elongated member, saidrigid elongated member having a center axis providing the axis ofpivotal motion of the first pivot assembly, a rigid inner support membercoaxially received on said stem portion substantially throughout thelength of the stem portion, a pair of cylindrical elastic damperelements coaxially received on opposite end portions, respectively, ofthe rigid inner tubular membmer, and a rigid outer tubular member havingopposite end portions coaxially received on said cylindrical elasticdamper elements, respectively, said rigid connecting means being rigidlyconnected between said rigid outer tubular member and a pivot assemblycoupled to said power unit, said rigid arm member being rigidlyconnected at one end thereof to said rigid outer tubular member of saidpivot support assembly.
 12. A power-unit support structure as set forthin claim 1, in which a pivot assembly coupled to said power unitcomprises a pair of bracket members securely attached to said power unitand formed with axial bores aligned with each other in a directioncoincident with the axis of pivotal motion of the second pivot assembly,a pair of outer rigid hollow cylindrical members coaxially received inthe axial bores in the bracket members, respectively, cylindricalelastic damper elements coaxially received in the outer rigid hollowcylindrical members, respectively, rigid inner cylindrical memberscoaxially received in the cylindrical elastic damper elements,respectively, a rigid elongated member having a stem portion extendingbetween the opposite ends of the rigid elongated member, said rigidinner cylindrical members being disposed coaxially around opposite endportions, respectively, of the stem portion of said rigid elongatedmember, said stem portion being passed through the axial bores in saidrigid inner cylindrical members, and a rigid outer tubular memberloosely received coaxially on the stem portion of said rigid elongatedmember and extending between said rigid inner cylindrical members, saidrigid connecting means being rigidly connected between said rigid outertubular member and said pivot support assembly.
 13. A power unit supportstructure as set forth in claim 2, which further comprises an additionalelastic damper element provided at the pivotal connection between saidfirst end of said swing member and said power unit.